Adhesive compositions containing organopolysiloxanes having vinyl groups, organopolysiloxanes having Si-bonded hydrogen atoms and catalysts which promote the addition of Si-bonded hydrogen atoms to the vinyl groups are described in U.S. Pat. No. 4,051,454 to Leiser et al. The vinyl containing organopolysiloxanes have a viscosity of at least 100,000 cp at 25.degree. C. and consist of diorganopolysiloxanes having from 0.1 to 1.0 mol percent of vinylmethylsiloxane units. These vinyl containing organopolysiloxanes contain no other vinyl groups other than the vinylmethylsiloxane units and triorganosiloxy groups and the remaining organic radicals are methyl and phenyl groups. The organopolysiloxanes having Si-bonded hydrogen atoms contain at least three Si-bonded hydrogen atoms and have from 0.01 to 1.7 percent by weight of Si-bonded hydrogen atoms. The silicon atoms which are not saturated with hydrogen atoms or siloxane oxygen atoms, are saturated with monovalent hydrocarbon radicals which are free of aliphatic unsaturation.
Adhesive compositions which form elastomeric or resinous compounds are also described in U.S. Pat. No. 3,527,655 to Ballard. These adhesive compositions are prepared from vinyl end-blocked organopolysiloxanes, organopolysiloxanes having Si-bonded hydrogen atoms, vinyl siloxane fluids having Si-bonded hydroxyl and alkoxy groups and catalysts which promote the addition of Si-bonded hydrogen atoms to vinyl groups.
When compared with the known adhesives, the compositions of this invention have several advantages over the adhesive compositions described in the above patents. For example, the organopolysiloxanes employed in the adhesive compositions of this invention are free of Si-bonded hydroxyl or alkoxy groups. The adhesives of this invention differ from and are superior to the adhesives described in U.S. Pat. No. 4,051,454 in that they contain organopolysiloxanes having a substantially lower viscosity. Thus, they need not be diluted with a solvent before they can be applied to substrates which are to be bonded together. This in turn substantially reduces the environmental problems generally associated with the presence of a solvent and also enhances their potential areas of application. Moreover, it is now possible to manufacture adhesives which are transparent and provide a means for glueing or coating solar cells. Surprisingly, substrates which are bonded together using the compositions of this invention adhere well even when the adhesive coatings do not contain any or contain fewer cross-linking sites than adhesive coatings prepared heretofore from the known adhesive compositions. Furthermore, the high degree of strength of the adhesive bond obtained from the compositions of this invention is indeed surprising since they do not contain the adhesion promoting organopolysiloxanes which are essential components in the compositions of U.S. Pat. No. 3,527,655.
It is therefore an object of this invention to provide adhesive compositions. Another object of this invention is to provide adhesive compositions which are transparent. Still another object of this invention is to provide adhesive compositions which may be used to prepare highly transparent laminated glasses. Still another object of this invention is to provide adhesive compositions having improved bond strength. A further object of this invention is to provide adhesive compositions which may be used for bonding unprimed substrates. A still further object of this invention is to provide adhesive compositions which may be used in the absence of a solvent.